Forced water cooling and evaporative cooling are known as methods of cooling such objects as a blast furnace and a heating furnace for heating steel ingots, steel plates and sheets, steel pipes, etc.
Forced water cooling is a method of cooling an object by circulating cooling water through a cooling pipe attached to the object to be cooled by means of a circulating pump. In this method, cooling water circulates in the liquid state and is substantially free from evaporation. Although the forced water cooling method is of wide application, it requires large quantities of cooling water, thereby not only necessitating associated facilities such as reservoirs and cooling towers, but also requiring much cost in operating and maintaining circulating pumps, cooling tower pumps, fans, etc. In addition, the temperature of cooling water at the cooling pipe inlet is as low as about 20.degree. C. to about 40.degree. C., resulting in a large heat loss due to cooling. Besides, the low temperature of cooling water at the cooling pipe outlet makes it difficult to recover heat from cooling water and to utilize recovered heat.
As a cooling method devoid of the above-mentioned disadvantages, evaporative cooling is widely used in Europe. This method comprises circulating cooling water through an evaporative cooling device comprising a cooling pipe fitted to an object to be cooled and a steam separator drum arranged above said cooling pipe, said cooling pipe and said steam separator drum being connected together by a downcomer and a riser, to cool said object to be cooled, separating steam generated from cooling water through heat exchange with said object to be cooled from cooling water by means of said steam separator drum, discharging said steam to outside the circuit, and replenishing cooling water from outside the circuit in an amount corresponding to the quantity of the steam discharged. This evaporative cooling method has many advantages over the above-mentioned forced water cooling method as follows:
(a) The amount of cooling water used is very small.
(b) Heat is recovered in the form of steam having a large heat capacity, thus permitting easy recovery of heat from cooling water and easy utilization of recovered heat.
(c) Since the temperature of cooling water at the cooling pipe inlet is as high as about 100.degree. C. to about 200.degree. C. heat loss due to cooling is small.
(d) It is unnecessary to provide associated facilities such as reservoirs and cooling towers.
Said evaporative cooling methods are classified into the forced circulation system and the natural circulation system. FIGS. 1 and 2 are respective schematic drawings of these systems. In both drawings, 1 designates a steam separator drum, 2 designates a downcomer, 3 designates an object to be cooled such as a blast furnace and a heating furnace, 4 designates a cooling pipe fitted to said object to be cooled, 5 designates a riser, and 6 (FIG. 1) designates a circulating pump.
In the evaporative cooling method by forced circulation, as shown in FIG. 1, cooling water is forcedly circulated by means of a circulating pump 6 arranged between a steam separator drum 1 and a cooling pipe 4. Therefore, even if a bend portion composed of an ascending part and a descending part is included in said cooling pipe 4, steam from cooling water does not stagnate, thus eliminating the possibility of causing "burnout" of said cooling pipe. When steam from cooling water is deposited on the inner surface of a cooling pipe, poor heat conduction of the cooling pipe is caused, and this leads to heat damage to the cooling pipe due to overheating. This phenomenon is called "burnout". Accordingly, the evaporative cooling method by forced circulation is adopted in applications where said bend portion must be provided in said cooling pipe as in the case of a walking beam furnace. However, this method needs said circulating pump 6, thus requiring much equipment cost and running cost. Moreover, since it is possible that said cooling pipe will be burned out if said circulating pump is stopped by an electric power failure and from other causes, a standby circulating pump driven by another power source must be provided.
On the contrary, in the evaporative cooling method by natural circulation, cooling water naturally circulates through the circulation force generated from the difference in density between the cooling water in a downcomer 2 and the cooling water in a riser 5, as shown in FIG. 2, requiring no circulating pump as in the case of the above-mentioned forced circulation system. More specifically, since the cooling water in said downcomer 2 is in the liquid state, it has a larger density than the cooling water containing vapor in said riser 5 in the gaseous and aqueous states, thus leading to natural circulation of cooling water due to difference in density. This natural circulation system has advantages in such points that equipment and running costs are low as compared with the above-mentioned forced circulation system, and further there is no possibility of the above-mentioned burnout even if an electric power failure and other situations take place. On the other hand, this method has a disadvantage that the above-mentioned burnout may be caused by deposition of steam films on said bend portion due to fluctuations of thermal load acting on said cooling pipe when said cooling pipe is provided with said bend portion. Therefore, this natural circulation system is adopted in applications where a horizontal beam is used and it is unnecessary to provide said cooling pipe with said bend portion composed of an ascending part and a descending part as in the case of a pusher furnace.
As mentioned above, although the evaporative cooling method by natural circulation is industrially most advantageous as a whole, such evaporative cooling methods by natural circulation have not so far been developed, which can be adopted in applications where said bend portion must be included in said cooling pipe.